Heat oven to 200°F. Line cookie sheet with cooking parchment paper. In medium bowl, beat egg whites with electric mixer on medium speed until soft peaks form. Gradually add granulated sugar, beating at high speed just until stiff peaks form.
In small bowl, mix powdered sugar and cocoa. Fold cocoa mixture, 1/3 at a time, into beaten egg whites. Spoon mixture into decorating bag fitted with star tip. Draw 1 1/2-inch diameter circle on white paper; place under parchment paper as a guide for piping rounds. Pipe mixture into twenty-four 1 1/2-inch rounds on parchment-lined cookie sheet. With back of teaspoon, make an indentation in each to hold filling. (Or, mixture can be spooned into dollops on parchment paper.)
Bake 1 to 1 1/4 hours or until crisp. Cool completely, about 10 minutes.
Meanwhile, in 1-quart heavy saucepan, heat whipping cream just to simmering over medium-low heat. Remove from heat; stir in chocolate with wire whisk until melted. Stir in butter pieces, a few at a time, until melted. Refrigerate until thickened, about 30 minutes.
Just before serving, spoon or pipe about 1 teaspoon filling into indentation of each meringue.